Preferential etching method for ferrous base metals

ABSTRACT

LIQUID COMPOSITIONS FOR ETCHING METALS AND ALLOYS AND METHODS OF ETCHING EMPLOYING THESE COMPOSITIONS ARE DISCLOSED IN WHICH SELECTED METAL STOCK OR METAL OBJECTS, PREFERABLY OF A FERROUS BASE, ARE PREFERENTIALLY ETCHED TO REMOVE PORTIONS OF METAL FROM THE WORKPIECE IN THE PRODUCTION OF FINAL ETCHED ARTICLES. THE COMPOSITION CONTAINS AS MAJOR INGREDIENTS A (1) PH REGULATOR COMPONENT COMPRISING AN ACIDIC PRECURSOR AND (2) AN UNDERCUT PROTECTOR COMPONENT COMPRISING CUPRIC IONS AND/OR OXYGENBEARING SALTS OF PHOSPHOROUS OR ARSENIC IN PARTICULAR COMBINATIONS AND PROPORTIONS WITH THE PH REGULATOR. A PREFERRED EMBODIMENT OF THE INVENTION UTILIZES A PARTICULAR COMBINATION OF THESE COMPONENTS WHICH INTERACT COMPATIBLY TO PRODUCE A SYNERGISTIC EFFECT IN THAT A VASTLY SUPERIOR RESULT IS OBTAINED AS COMPARED TO THAT WHICH WOULD BE EXPECTED.

United States Patent O 3,758,351 PREFERENTIAL ETCHING METHOD FOR FERROUS BASE METALS Walter J. Striedieck, Port Matilda, and George Daniel Woodring, Bellefonte, Pa., assignors to Chemcut Corporation, State College, Pa.

N Drawing. Continuation-impart of abandoned application Ser. No. 749,591, Aug. 2, 1968. This application July 15, 1971, Ser. No. 163,036

Int. Cl. C23b 3/02 US. Cl. 156-18 7 Claims ABSTRACT OF THE DISCLOSURE Liquid compositions for etching metals and alloys and methods of etching employing these compositions are disclosed in which selected metal stock or metal objects, preferably of a ferrous base, are preferentially etched to remove portions of metal from the workpiece in the production of final etched articles. The composition contains as major ingredients a (1) pH regulator component comprising an acidic precursor and (2) an undercut protector component comprising cupric ions and/ or oxygenbearing salts of phosphorous or arsenic in particular combinations and proportions with the pH regulator. A preferred embodiment of the invention utilizes a particular combination of these components which interact compatibly to produce a synergistic effect in that a vastly superior result is obtained as compared to that which would be expected.

This is a continuation-in-part of application Ser. No. 749,591, filed Aug. 2, 1968, now abandoned.

BACKGROUND OF THE INVENTION For many years, etching of metal surfaces such as, for example, in the making of metal printing surfaces, has been done by coating a plate of an acid soluble metal, such as zinc and copper or alloys thereof, with a light sensitive coating. This coating was then subjected to electromagnetic radiation, such as a strong light through an image-bearing film transparency in contact with the coating or by focusing the image of the transparency thereon. The portions of the coating exposed to the light would harden and become etch resistant, while unexposed portions of the coating could subsequently be removed to reveal bare metal for etching, leaving said acid etch resistant coating as an image pattern on the plate. This image is sometimes further hardened by heating to make it more durable. The plate so prepared was then contacted with an etching solution, which attacks the bare metal portions of the metal plate not protected by the harden coating. As etching progresses, the etchant not only attacks the unprotected metal but also the side walls which begin to form about the periphery of the image areas. Accordingly, etching action also proceeds laterally and undercuts the coated image areas, thus producing a final article which, among other defects, no'longer conforms with the original image, and is, therefore, unsuitable as a printing surface.

In the past, such undercutting was controlled by periodically interrupting the etching action and mechanically banking a deposit of a hardenable resinous powder on the formed side wall of the image, then heating to convert the powder into an etchant resistant barrier. This method of controlling undercutting, called powdering, was extremely time consuming and expensive and, therefore, in recent years has been replaced by a so called powderless v process, wherein an organic film is formed on the side walls automatically during the etching operation.

The old powder process and the more recent powderless process of etching are described in detail in, for

example, US. Letters Patents 2,640,767, 2,828,194, and 3,152,083.

While the powderless process has been very successful and widely used, particularly in the preparation of metal relief printing plates, even to the extent of obsoleting the old powder method, it suffers the serious limitation of only being applicable to non-ferrous base metals such as zinc, copper and magnesium, and their alloys. Therefore, notwithstanding that iron and steels comprise by far the lowest cost most extensively used base metal throughout the world, ferrous base metals have not been available for the many applications which a ferrous base metal powderless etching process could make available such as listed herein before. 7

DESCRIPTION OF THE INVENTION This invention relates to new and improved powderless etching baths and to improved methods of powderless etching and particularly to novel etching baths and etching methods for use in the selective or preferential etching of a variety of things such as, for example, massive pieces or flat or curved metallic foils, sheets and plates, in the making of articles such as computer print drums, type bars, molding dies and printing plates. The invention also relates to and is useful in the field of chemical machining of metals for making such articles as templates and other blanked, contoured or perforated objects.

Accordingly, the present invention, in providing a powderless type method for etching ferrous metals represents an extremely significant and pioneering contribution to the art, in that now the basic metal of industry can be chemically processed or etched powderlessly to produce not only printing plates but numerous other articles which were heretofore only available for production by stamping or punching or mechanical machining. Moreover, the present invention has the inherent advantage of producing a totally unstressed and burr-free part or article of a ferrous base metal. It also has the added advantage of being able to powderlessly etch magnesium base metals as well as ferrous base metals.

Since the present invention has significant utility in the preparation of relief letterpress printing plates, this application has been used as a convenient means or vehicle to describe the invention and show its utility. It should be understood, however, that this invention is by no means limited to photoengraving, and actually has its greatest utility and applicability in other fields such as e.g. chemical machining and in the making by etching of mass quantities of parts and in the production of other articles such as molding dies, templates, and computer print drums and type bars, wherein either complete penetration or perforation of the workpiece if obtained by the etching action, or only a contouring or shaping effect. This invention does not relate to methods or compositions of chemical conversion coatings for the preparation or passivation of metal surfaces.

For use in describing the quality of results obtained from the present invention, a convenient measure is used herein called etch factor to indicate the extent of lateral attack in the direction of the image side wall. This measure is well known in the photoengraving art and as used herein is defined as the ratio of the depth of etch to the extent of undercut. The depth of etch is measured in a direction perpendicular to the etchant-resistant (simply called a resist) coating from the surface to the bottom of the area most deeply etched adjacent to the image. The extent of undercut is the distance parallel to the coating from the original edge of the image to the top edge of the remaining metal.

An etch factor of about three is the present optimum value attainable (without any artificial side wall protec- 3 4 tion) on ferrous metals prior to the present invention. In a reservoir. The coating was oven hardened and photomost if not all applications, however, an etch factor of printed by contacting the coated surface with an image or even 25 or more is required. Thus, prior to the bearing negative through which a strong light beam was present invention, for such applications ferrous metals directed. The portions of the coating exposed to the light have been excluded from powderless etchant use. With 5 subsequently became resistant to the developing solution the advent of this invention, this limitation no longer which removes the unexposed coating. A final oven hardexists since etch factors of from 4 to more than 40 can ening readied the plate for etching. This plate was etched be attained together with other good and desirable charfacedown in a one liter Chemcut spray etching machine acteristics of etch quality such as, e.g., cleanliness of the equipped with Steinen number SSM-l21-SQ nozzles, and etched surface (little or no pimple-like defects) rapidity 1O operated at 120 F. with a spray pressure of 3 p.s.i. The

and uniformity of etching and a desirable shoulder angle test piece was held in position on a magnetic plate mountor profile. Moreover, the present invention provides a ed within the etching apparatus. The test piece was etched rapid etch rate as will be shown and described hereinfor the time noted below and then washed and examined after. under a microscope for determination of etch factor.

Briefly, the present invention comprises compositions The etching bath used in this example consisted of and methods for etching ferrous and magnesium metal the following ingredients in the amounts indicated: surfaces. These compositions are aqueous solutions with HN0 1,42) l 3 a hydrogen ion concentration of 0.01 to 9 grams per liter, C (N() (19% H 0) g11'1s 100 a lateral etch inhibitor comprised of cupric ions in a N g-IP0 (anhydrous gm 10 concentration of 3-140 grams per liter and an additional 20 E 0 l 937 lateral etch inhibitor comprised of phosphate ions, in the /1 ,05 range of 2 to 365 grams per liter, and/or arsenate ions, N0 g./l 55.65 in the range of 1 to 365 grams per liter. P0 g,/1 6,7 Within the foregoing range, the two preferred bath c /1 27.2 compositions (to charge a 12 gallon etcher) are: Etch depth in .015 (1) 11.9 gallons water, 2.9 gallons 85% H PO and 14 An etch factor of 15 was obtamed' pounds copper oxide (hereinafter sometimes referred EXAMPLE 2 to as a pp Oxide bath) and The procedure of Example 1 was followed using the g l Wafer, gallons 35% a o 144 etching bath and operating conditions set forth below:

tan erine settl rs: resists; e Ma bath) Cu(NO (19% H O) gms 200 Na HPO (anhydrous cp.) gms 550 The former is preferred for high carbon alloy steels, H O ml 250 such as 52-100 (A485-3), 4720, 4730 and 1030, while Operating machine temperature F 120 the latter is preferred for low carbon steels such as SAE Spray pressure p.s.i.. 10 1018, A181, 1018, 1006, 1008, 1010, 1015 and 1016. The Etching time mins 20 latter bath (nitric acid) may not in some cases etch the H+ g./l 7.9 high carbon or alloy steels satisfactorily. NO,- g./l 599.5 The composition of the foregoing alloys and an indica- PO; g./l 367 tion of the source of the specifications therefor are set Cu++ g./l 54.4 forth in the following table: Etch depth in .025

TABLE 0 Si Mn Cr Mo Ni Pm. Sm.

E 52 100 (A181) (SAE-same) .95-1. 10 .20-.a5 .25-.45 1.30-1.00 .025 .025 A4B53(ASTM A- .0 1. 5 .025 4720 (AISI) (SAE same)- 4730 (A s 1030 (A 1018 (AISI) (SAE-same). A181 A TM) 100s (AISI) 100s (AISI). 1010 (A181). 1015 (AISI). 1016 AIM In the method of the present invention, the baths de- 55 scribed herein are sprayed at high pressure and close An etch factor of 12 was obmmed' range, while at an elevated but moderate (70-140" F., EXAMPLE 3 preferably 70-90 F.) temperature, onto a metal plate to be etched. Ordinarily, the plate will include some maski procedure of Example 1 Ye followed usmg the ing such as patterned resist coating, to prevent etching of etchmg bath and operatmg commons Set forth below: preselected parts of the metallic surface. In many cases, this method is used preferably in stages, i.e. the pattern "3 :2 is partially etched, the etchant and inhibitor are removed 2110 powdezr) g 75 and the process is repeated. H PO (Sp gr 25 The present invention may be better understood by 4 90 reference to the following detailed examples and further ir "'5 o 7 description thereof. 'In all of the following examples, un- S P l g mac me empera um 125 less otherwise indicated, the steel etched was either 1008 z pressure 13 or 1010 steels. 611mg tune --m1ns 2 EXAMPLE 1 H+ g./l 2.95 A 1008 steel plate 2" x 2" was throughly cleaned by 146 use of a vapor degreaser and by scouring with a cleanser i; grade pumice. The surface was air-blast dried. Coating Cu 1 Etch depth 1n .014

with a photo-resist such as Kodak KMER was accomplished by slow mechanically controlled withdrawal from An etch factor of 10 was obtained.

5 EXAMPLE The procedure of Example 1 was followed using the etching bath and operating conditions set forth below:

HNO (sp. gr. 1.42) m1 Cu(NO (19% E gms 400 Na I-I'PO (anhydrous cp.) gms 100 H2O ml Operating machine temperature F 120 Spray pressure p si 8 Etching time i 'mins '10 H e g./l .16 NO; g./l 225 P0 g./l 67 Cu++ g'./l 109 Etch depth in 025 An etch factor of 12 was obtained.

EXAMPLE The procedure of Example 1 was followed using the etching bath and operating conditions set forth below:

'HNO (sp. gr. 1.42) ml Cu(NO (19% H O) gms 100 NaHPO (anhydrouscp) .'.gmS 5 H2O ml Operating machine temperature F 120 Spray pressure p.s.i 5 Etching time mins .20 H+ g./l .16 NO; g./l 63.5 P0 7 g./l 3.3 Cu++ g./l 27.2 Etch depth in .022

An etch factor of 11 was obtained.

EXAMPLE 6 The procedure of Example 1 was followed using the etching bath and operating conditions-set forth below:

HNO (sp. gr. 1.42) ml 3 Cu(NO (19% B 0) gms 110 Na I-IAsO (heptahydrate) gms 25 H O ml 920 Operating machine temperature F 120 Spray pressure p.s.i 7 Etching time mins 10 H+ g./1 .05 N0 g./l' 62.1 AsO.;, g /l 11.2 Cu++ g./l 30.2 Etch depth in .013

An etch factor of 13 was obtained.

EXAMPLE 7 The procedure of Example 1 was followed using the etching bath and operating conditions set forth below:

HNO (sp. gr. 1.42) ml Cu(NO (19% H O) gms' 500 NaHAsO (heptahydrate) gms 600 H O ml 100 Operating machine temperature F 125 Spray pressure p.s.i 2 Etching time mins 15' H+ g./1.. 7.9 NO; g./l 761 AsO; g./l 365 Cu++ g /l 136 Etch depth in .015

An etch factor of 10 was obtained.

6 EXAMPLE 8 'The'procedure of Example 1 was followed using the etching bath and operating conditions set forth below:

HNO (sp. gr. 1.42) m1. 4 Cu(NO (19/ Hi0) gms 45 Na HAsO (heptahydrate) gms 15 H O ml.. 966 Operating machine temperature F 125 Spray pressure p.s.i 10 Etching time mins.. 20 11+ g./l .07 N03 I g./l 28.0 AsO 'f g./1 6.7 Cu++ g./l 12.2 Etch depth in .015

An etch factor of 10 was obtained.

EXAMPLE 9 The procedure .of Example 1 was followed using the etching bath and operating conditions set forth below:

,HNO (sp. gr. 1.42) ml 10 Cu(NO (19% H O) gms. Na HASO (heptahydrate) gms 5 H O ml 937 Operating machine temperature F Spray pressure p.s.i l0 Etching time mins 15 H g./l .16 NO g./l 63.5 AS0 7 g./l 2.2 Cu++ g./l 27.2 Etch depth in .009

An etch factor of 18 was obtained.

EXAMPLE 10 The procedure of Example 1 was followed using the etching bath and operating conditions set forth below:

EXAMPLE 11 The procedure of Example 1 was followed using the etching bath and operating conditions set forth below:

HNO (sp. gr. 1.42) rn 1 500 Cu(NO (19% H O) grns 500 Na NAsO (heptahydrate) gms 300 Na HPO (anhydrous cp.) gms 300 H O ml 100 Operating machine temperature F 125 Spray pressure p.s.i 2 Etching time mins 10 H+ g./l 7.9 N0 g./l 760 PO; g./l 202 ASO4 3 g./l 134 Cu++ g./l 137 Etch depth in .018

An etch factor of 10 was obtained.

7 EXAMPLE 12 The procedure of Example 1 was followed using the etching bath and operating conditions set forth below:

HNO (sp. gr. 1.42) ml 6 C11(NO (19% H O) sgms" 50 Na HAsO (heptahydrate) gn1s Na HPO (anhydrous cp.) gms 10 H O ml 960 Operating machine temperature F.. 125 Spray pressure p.s.i 8 Etching time mins 10 H'' g./l 0.1 NO g./l 32.7 AsO g./l 4.5 PO g./l 6.7 Cu++ g./l 13.6 Etch depth in .012

An etch factor of 12 was obtained.

EXAMPLE 13 The procedure of Example 1 was followed using the etching bath and operating conditions set forth below:

An etch factor of 11 was obtained.

EXAMPLE 14 A high etch factor, of the order of 30 to 40, may be obtained on low carbon steels in the mode for carrying out the present invention set forth in the following example:

The procedure of Example 1 is followed using a spray etching machine operated at 125 F. and a spray pressure of 8 p.s.i. An etching time of ten minutes is employed using an etching bath consisting of the following ingredients in the amounts indicated:

Cu(NO (19% H O) gms 300 Na HAsO (heptahydrate) gms 125 H P0 85% (sp. gr. 1.70) ml 70 H 0 ml 720 PH g./l .21 N0 g./l 161 ASO4 3 g./l 56 P0 g./l 98 Cu++ g./l 82 Etch depth in .025

An etch factor of 35 was obtained.

EXAMPLE For the embodiment of the invention comprising hydrogen ion, cupric ion and phosphate ion, a very desirable mode for carrying out the present invention is set forth in the following example:

For the embodiment of the invention comprising hydrogen ion, cupric ion and arsenate ion, the best mode currently known to us for carrying out the present invention is set forth in the following example:

HNO (sp. gr. 1.42) ml 10 Cu(NO (19% H O) gms Na HAsO (heptahydrate) gms 10 H O ml 1000 Operating machine temperature F Spray pressure p.s.i 7 Etching time mins 20 H+ -g./l .16 NO; g./l 63.9 AsOp g./l 4.5 Cu++ g./l 27.2 Etch depth in .025

An etch factor of 12 was obtained.

EXAMPLE 17 The procedure of Example 1 is followed using the etching bath and operating conditions set forth below:

CuO gms 50 H PO 85% (sp. gr. 1.70) ml 125 H O in an amount sufiicient to make 1 liter. Operating machine temperature F 125 Spray-to-work distance in 4 Spray pressure p.s.i 5 Etching time mins 10 H+ g./1 3.75 PO; g./l 176 Cu+'- g./l.. 40 Etch depth in .012

An etch factor of 36 was obtained.

EXAMPLE 18 A highly desirable bath, insofar as etch factor is the criteria for determining desirability is set forth in the following example:

An etch factor of 44 was obtain d.

9 As previously indicated, the bath and the process. of the present invention may be employed particularly in the field of chemical machining of metals where a vertical side wall is of greater importance than a high etch factor. The following example illustrates such use of the invention and resulted in nearly vertical side walls.

Steel, 1010 CRS.

An etch factor of 12 was obtained.

The sample which was etched in this example was a small motor lamination. This part was etched from both sides entirely through the original metal plate-unlike the usual photoengraving examples which seek to emboss the surface. After the part was etched to the breakthrough point, it was further etched to remove the foot which is a result of the powderless protective process. Ideally, the product should have as nearly vertical side walls as possible. This example does present an acceptable side.

wall profile.

EXAMPLE 20 An etching bath and process for the etching of magnesium photoengraving alloys (such as 3% aluminum,

1% zinc, 96% magnesium) are set forth in the following example:

Cu(NO (19% H O) gms 300 Na HPO (anhydrous cp.) gms 75 H PO 85 (sp. gr. 1.70) ml 75 H O in an amount sufiicient to make, 1 liter.

Operating machine temperature F 135 Spray pressure p si 1 Spray-to-work distance in 9 Etching time mine 20 H 2 1 2.3 NO E/i-.. 161 PO; g/l 156 Cn++ g /l 82 Etch depth in .030

An etch factor of 15 was obtained.

EXAMPLE 21 An etching bath and process for the etching of 1010 cold rolled steel are set forth in the following example:

FeCl (w./ 20 oz./gal. copper) ml 900 HNO (sp. gr. 1.42) ml 180 Na HPO (anhydrous cp.) gr 10 Operating machine temperature F" 120 Spray pressure p.s.i 2 Etching time "mins-- 10 H+ g /l 3.0 NO;- g/L- 177 P 2 /1 6.7 Cu++ g/l 135 Etch depth in .006

An etch factor of 17 was obtained.

10 EXAMPLE 2;

. The procedure of Example 1 is followedusing the etching bath and operating conditions set forth below:

Na- HPCM, (anhydrous cp.) gms 10 HNO (sp. gr. 1.42) ml 50 H O in an amount sufiicient to make 1 liter.

Operating machine temperature F" 125 Spray pressure p.s.i 8 Spray-to-work distance in.. 6 Etching time ..mins 15 H+ g./l .8 N0 g /l 49 P047 g /1 6.7 Etch depth in .006

An etch factor of 5 was obtained.

EXAMPLE 23 The procedure of Example 1 is followed using the etching bath and operating conditions set forth below:

Na HAsO (heptahydrate) gms.. HNO (sp. gr. 1.42) ml 75 H O in an amount sufficient to make 1 liter.

Operating machine temperature -F Spray pressure p.s.i 6 Etching time mins 15 H+ g./l 1.1 8 N0; g/L- 74 AsO. g./l 67 Etch depth in .006

An etch factor of 9 was obtained.

It will be noted that copper is not present in either of the two foregoing baths and the reduced etch factors are thought to be a result thereof. These baths are outside the scope of the present invention and these examples are included herein for comparison only.

EXAMPLE 24 The following example was etched in an experimental 6 liter paddle etcher such as is widely used in the photoengraving art:

H PO g./l 65 Cu(NO =(19% H O) g./l 250 Na I-IAsO (heptahydrate) g./l.... 60 Etching time mins 8 11+ g /l 2.0 NO;- g/l 134 ASO4 3 g /l 27 PO; g /1 92 Cu++ g /l 68 Etch depth in .018 Paddle speed r.p.m 750 Bath temperature F 120 An approximate etch factor of 15 was obtained.

EXAMPLE 25 The following example shows the addition of Cu++ to the bath in the form of CuCO CuCO gmq 100 H PO ml 250 H O 1 (balance)... 2 Etching time mins 15 H g /l 4 P0 5; /l 176 Cu g /l 26 Etch depth in .014 Spray pressure p si 4 Temperature F An etch factor of 7 was obtained.

bility of etching a mild steel plate. The etch factor given is the optimum figure thought to be obtainable with this etchant.

FeC1 (42 B. circuit etching grade) l 1 Etching time mins 10 Spray pressure p.s.i 15 Temperature F 130 H+ g./l .05 Fe+ g/l 214 C1 g./l 457 A maximum etch factor of 3 was obtained.

The following examples are indicative of the lower temperatures at which the etching process of the present invention may be effected. It should be noted that the preferred temperatures are in fact in this lower range.

EXAMPLE 27 Cu(NO 19% kg 1 Na HAsO (heptahydrate) gms 250 H PO ml 300 H O l. (balance) 5 Temperature F 100 Spray pressure p.s.i 4 Etching time mim 15 H+ g./1 1.8 NO-;, g/l 107 AsO g /l 22 PO; g./l 84 Cu++ g /l 54 Etch depth in .006

An etch factor of 6 was obtained.

EXAMPLE 28 Cu(NO 19% kg 1 Na HAsO (heptahydrate) gms 250 H PO m1 300 H O 1 (balance) 5 Temperature -F 110 Spray pressure p.s.i 40 Etching time mins 5 H g./l 1.8 AsO, g./l 22.3 N g./l 107 P g./l 84.2 Cu++ g /l.. 54 Etch depth in .006

An etch factor of 6 was obtained.

EXAMPLE 29 The following example shows the results of etching with a high carbon content steel. The steel used contained 0.95% carbon.

CuO gms 100 Cu(NO (19% H O) gms 100 H PO ml 300 H 0 1 (balance) 2 Temperature F 120 Spray pressure p si 6 Etching time mins 15 H+ g./l 1.8 N0 g l 27 P0,, g ll-.. 84 Cu++ g./l 53.6 Etch depth in .023

An etch factor of 23 was obtained.

EXAMPLE 30 The procedure of Example 1 was followed using the etching bath and operating conditions set forth below: This formulation is representative of an elevated etch rate of which the present invention is capable, the etch rate being in excess of .003" per minute.

H PO (sp. gr. 1.70) rnl 70 Cu(NO (19% H O) gms 300 Na HAsO (heptahydrate) gms H O in an amount sufiicient to make 1 liter.

Temperature F Spray pressure p.s.i 5 Etch time mins 7 Spray-to-work distance in 4 H g./l 2.1 N0 g./l 161 AsO g./l 44.6 P0,; l 95.0 Cu++ g./l 82 Etch depth in .024 Undercut in .00075 An etch factor of 32 was obtained.

EXAMPLE 31 The foregoing examples, except Example 29', illustrate undercut inhibited etching of low carbon steels. Various special problems arise in the application of the compositions and methods of the present invention to etch, with satisfactory side wall shapes, high carbon or alloy steels. The most effective (and in some cases the only effective) etchant for these steels found to date is the copper oxide bath described above as preferred etchant 1 and also exemplified in Example 17, above.

Further, these high carbon or alloy steels must be in the normalized condition, as described more fully hereinafter.

Thus, by way of specific example, several samples of high carbon alloy steels, namely 52-100 (A485-3), 4720, 4730 and 1030, purchased in the normalized condition, were surface ground to a smoothness of 12 microinches, degreased, cleaned and covered with a patterned resist film in a conventional manner. The pattern area not covered with resist was primarily indicia, such as letters, numerals and symbols.

The etchant was prepared by adding 14 pounds copper oxide, with continuous agitation, to 2.9 gallons 85% phosphoric acid and diluting the solution with 11.9 gallons water. In an etching machine with a 12 gallon capacity, this etchant was sprayed at about 73 F. and under a pressure of 25 p.s.i., through a conical spray nozzle, commercial designation, Steinem 2688, with the Work pieces at a distance of 4 inches.

An etch factor of 10:1 and an etch rate of /2 mil/ minute were obtained with good conformance of etch shape to resist indicia characters. Among other qualita- EXAMPLE 32 Ten pounds of cupric nitrate was dissolved in 9.6 gallons of water, to which was added 144 milliliters 42 B nitric acid and 2.4 gallons 85% phosphoric acid. This etchant was charged to an etcher having a 12 gallon capacity and used under conditions practically identical, except for spray pressure to those used in Example 31 to etch samples of several low carbon steels, namely SAE1018, A151, 1018, 1006, 1008, 1010, 1015, and 1016, all received and used in the as rolled condition and all prepared as were the samples in Example 31. With a nozzle pressure of 4 p.s.i., an etch factor of 5:1 with an etch rate of mil/minute and relatively narrow shoulders (straight perpendicular side walls) were obtained. Increasing the nozzle pressure to 15 p.s.i. resulted in increasing the etch rate to 1 mil/minute and decreasing the etch factor to 3:1. The shoulders or side walls remained straight, however.

This is consistent with experience in the powderless etching art generally which indicates that high pressures tend to reduce the amount of coating which is responsible forside wall protection. The result is steeper side walls as well as some sacrifice in etch'factor. correspondingly, lower pressures tend to produce broader side wall configurations with a general tendency toward slightly improved etch factors.

The foregoing preferred baths set forth in Examples 31 and 32 are calculated to have the following anionic contents:

H+ PO-FB Cu+ Example 31, g./l 8. 55 273. 6 89. 5 8. 63 271. 7 26. 6

Example 32, g./l

The ranges of ionic concentration established by all of the foregoing examples therefore are as follows:

H+ 0.5-8.63 P0 7 3 .3-357 Cu+ 26-109 H+ ca. 8.6 P0 270-275 Cu+ 25-90 As is evident from the foregoing examples, the powderless etching baths of this invention contain two major classes of components: (1) pH regulators, and (2) lateral etch minimizing agents.

A variety of pH regulators may be used to supply or remove hydrogen ions to maintain the bath preferably over a range of 0.1 to 2.0. Although the examples show only the use of nitric and phosphoric acids which are preferred, it must be understood that the pH regulators are not restricted to these materials but may also be other acids, such as hydrochloric, hydrofluoric, perchloric and sulfuric (used either singly or in combinations); acid salts, such as ferric chloride, ferric nitrate,

aluminum nitrate and zinc nitrate; or buffer salts such as ammonium citrate, ammonium acetate and sodium tartrate.

The lateral etch minimizing agents which have been found effective are of two types: (1) those which produce a metallic deposit on the workpiece, such as copper; (2) those which produce a thixotropic gelatinous, non-metallic coating on the surface, such as phosphates and arsenates. The above described lateral etch minimizing agents are used in combination. The metallic coating former is copper ion in concentrations ranging from 3 to 140 grams per liter. Although the sources of copper ions shown in the examples are restricted to cupric nitrate, cupric oxide and metallic copper, it must be understood that other salts may also be successfully used such as cupric carbonate, cupric sulfate, cupric hydroxide, cupric chloride and cupric acetate.

The inorganic gelatinous coating formers are essentially phosphates and arsenates. Phosphorous compounds which have been found to be satisfactory sources of phosphate ion are phosphoric acid, the mono, di and tri-sodium orthophosphates, sodium hexametaphosphate, tetra-sodium pyrophosphate, phosphorous trioxide, phosphorous pentoxide, zinc phosphate, lithium phosphate, mono-, diand tri-sodium phosphates, mono-, diand tri-ammonium phosphates and manganese phosphate. Any phosphorous compound which reverts to orthophosphate ion under operating bath make-up conditions to provide phosphate ion concentrations in the range of 2 to 365 grams per liter can be expected to perform satisfactorily.

Arsenic compounds which have been found to be acceptable sources of arsenate ion are mono-, diand trisodium arsenates; mono-, diand tri-potassium arsenates; sodium, potassium and ammonium meta-arsenites; arsenic oxide; mono-, diand tri-ammonium arsenates, arsenous oxide; and basic copper arsenate. Any arsenate compound which reverts to orthoarsenate ion under operating bath make-up conditions to provide arsenate ion concentrations in the range of 1 to 365 grams per liter can be expected to perform satisfactorily.

Practical usage of these baths demands that operating temperatures be selected to produce the most rapid etch rates possible while permitting adequate control of the process. While it is possible to get etching action at temperatures as low as 40 F., the most practical operating temperatures range from 70 to 140 F., with 70 to F. being preferred. Satisfactory etching can be attained at higher temperatures up to the boiling point, but practical upper temperature limits are governed by the corrosion resistance and temperature limitations of the equipment in which the bath is being used.

Many of the baths of the present invention, particularly those of the copper oxide variety, require agitation, and sometimes severe agitation, to keep them in sprayable form.

For proper operation of this bath, it is necessary to apply the etchant forcefully to the workpiece surface. The direction of force should be as nearly perpendicular as possible. There are several methods for impinging the etchant on the surface of the workpiece. For example: (1) Spray etching has been the method most widely employed with these baths and offers the most satisfactory control of impingement force and direction. (2) Paddle etching which, in the prior photoengraving art, has won highest acceptance and has also proved satisfactory with the baths of this invention, (3) It is possible to apply etchant forcefully to the surface of a workpiece by methods other than these two; such means may include ultrasonic agitation, for example. Whichever method is employed, the agitation of the entire mass of the bath must be sufficiently vigorous to prevent the thioxotropic baths from becoming gelled. The principles of selection of noz zle configuration, spray pressures and impingement forces are no different from those utilized with conventional baths by persons skilled in the art. Similarly, side wall etching, even though substantially inhibited, will occur to different degrees depending on specific process conditions. As in conventional powderless etching, this can be compensated for by making the image slightly smaller than the actual size desired in the finished etch.

In some cases, the etching process of the present invention may slow considerably or stop altogether if continued for an extended period of time. In such cases and also in other special cases where certain etch geometry is desired, the method of the present invention may be used repeatedly or in conjunction with other etch processes, such as inhibited ferric chloride etching, conducted serially. Between steps of these multi-step procedures, the workpiece is thoroughly cleaned and all inhibitor and etchant is removed.

The mechanism responsible for the desirable results obtained by incorporating phosphate and/ or arsenate ions in an etching bath containing cupric ions is not fully understood. Very shortly after the workkpieoe metal is exposed to the etching action, the bath also dissolves ions of the metal being etched, and a number of chemical changes and reactions are initiated involving all these ions. While the exact nature of these reactions is not known, certain physical changes have been noted. For example, with the baths containing phosphate or arsenate there is a gradual change to a thixotropic state, so that with a particular type of agitation there is a progressive thickening of the bath as the etching proceeds. The precise role of the cupric ion in these reactions is also not known, but we have determined that the synergistic effect of the present invention is not obtained if the cupric ion is omitted. Likewise, the effect is not obtained if no phos phate or arsenate ionis included.

,From the examples it will be noted that the aqueous etching baths of the present invention are characterized by the presence of at least one pH regulator selected from the group consisting of inorganic acids and acid salts which in water solution dissociate to form hydrogen ions; and at least one lateral etch minimizing agent selected from the group consisting of chemical elements, salts, acidsand acid anhydrides which in acidic water solution dissociate into ions which are members of the group consisting of copper, phosphate and arsenate ions. A satisfactory concentration range for the hydrogen ion is from about 0.01 grams per liter to 8 grams per liter and a satisfactory concentration range for the cupric ion is from about 3 grams per liter to:.about 140 grams per liter. A satisfactory concentration range for phosphate ions is from about 2 grams per liter to about 365 grams per liter, and for arsenate ion itis from about 1 gram per liter to about 365 grams per liter.

As far as is known the upper limit of phosphate and arsenate is set only by the solubility of these ions. High copper content will lead to copper plating throughout the etch surface and substantially inhibit all etching, High copper content may be compensated for by ferric ions in solution, however. For this purpose, and sometimes to catalyze the initiation of the etching process when it is slow in starting for some other reason, iron ions or a piece of iron may be added to the etchant bath. Alternatively, a new bath may be seeded with a portion of an old bath for the same purpose.

As indicated in Example 31 above, when high carbon or alloy steels, such as 52-100 (A485-3) 4720,4730 and 1030 are used, the steels must generally be in normalized condition.

Thus the steel is neither in the full hard nor full soft condition. Steels which have been hardened, such as by rolling or roll straightening, may be normalized by heating to a temperature about 100 F. above the critical temperature range for that steel and then cooled to below that range in still air. This produces a generally fine pearlite structure which is etchable in accordance with the present invention. Hardened steel has a martensitic structure which is not etchable, with proper side wall control and undercut inhibition, by the compositions and methods of the present invention.

No comparable problem is found in low carbon steels which may generally be etched in the as produced or annealed condition. With all steels, however, care must be taken to ensure that the surface to be etched is in proper condition. Any machining with cutting tools must be such that no cold flow or working of the metal occurs to any great depth. Cutting to a depth of about .010 inch 16 a. that the correspondng acetates shall be included since the active portion of the molecule gives the desired ions on dissociation in aqueous solution.

We claim:

1. A method of preferentially etching surfaces of certain ,carbon steels or low alloy steelsv containing molybdenum, chromium or nickel or mixtures thereof which comprises: providing a liquid aqueous etching composition comprising (1) at least-one pH regulator selected from the group consisting of nitric, phosphoric, hydrochloric and sulfuric acids which in water solution dissociate to form hydrogen ions in a hydrogen ion concentration within the range of about 0.01 to about 9 grams per liter of bath, (2) at least one lateral etch-minimizing agent selected from chemical precursors which in acidic solution .forms cupric ions in a concentration of from about 3 to as much as about 140 grams per liter of bath, and (3) an additional lateral etch minimizing agent selected from the group consisting of soluble inorganic phosphates and" arsenates which, in acidic solution, form at least one of the following, arsenate ions in a concentration of from about 1 to as much as about 365' grams per liter of bath and phosphate ions in a concentration. of from about 2 to as muchas about 365' grams per liter of bath, the total concentration of said cupric, arsenate and phosphate ions when used in combination being limited to a compatible amount to permit reasonable con-, tact of the composition with the steelworkpiece to be etched and to produce the desired quality of etch, the balance of the bath being essentially water, forcefully contacting the etching composition, at a temperature within the range of from about 70 to about 140 F., with at least a portion of the steel surface to be etched, said surface having an etch-resistant pattern thereon, continuing said contact of the etchant composition and steel surface for a period to obtain the desired degree of etching effect, whereby the areas of said surface not covered by said etch-resistant pattern are etched as a protective coating, formed from said cupric and said phosphate or arsenate ions in said composition is deposited on thesidewalls of said etch-resistant pattern.

2. The method of claim 1 wherein said composition temperature is approximately 7 0-90 F.

3. A method of preferentially etching low carbon steels, with inhibited undercut and side wall etching, said method comprising forcefully contacting an etching com,- position, said composition consisting, per gallon of solution, of

42 Baum nitric acid=12 milliliters 85% phosphoric acid=.20 gallon .833 pounds copper nitrate Balance=water at a temperature in the range of 70-140 F. with :at, least a portion of the surface of the low carbon steel to..be etched, said surface having an etch-resistant pattern-there;

on, continuing said contact of the etchant composition is considered the acceptable maximum. Generally the surto these particular methods of etching orchemical machining or chemical milling. Where the term inor ganic is used in the specification and claims, it is intended phosphoric acid=0.24 gallon 1.17 pounds per cupric oxide Blalance=water at a temperature in the range of 70-140 F. with at least a portion of the surface of the high carbon steel to be etched, said surface having an etch-resistant pattern thereon, continuing said contact of the etchant composition and workpiece for a period to obtain the desired degree of etching effect.

17 5. A method, as recited in claim 4, wherein said addi- FOREIGN PATENTS tional lateral etch minimizing agent is a soluble inorganic 92 384 12/1961 Denmark ,5 2 phosphate.

OTHER REFERENCES 6. A method, as recited in claim 5, wherein the cupric n concentration is in the range 25-90 grams P liter- 5 Striecher: Phosphatization of Metallic Surfaces, pp.

7. A method, as recited in claim 6, wherein said phos- 51-59 of Metal i i hi August 1948,

phate ion concentation is in the range 270-275 grams per ch khl m t 1,; pp 706-709, Jzvest Vysshi Kh liter, and said hydrogen ion concentration is approxi- Ucheb Zavedenii Khimi Khim. TekhnoL, vol. 14 (1961).

1 mate y 8 6 grams per 10 GEORGE F. LESMES, Pnmary Examiner References Cited R. J. ROCHE, Assistant Examiner UNITED STATES PATENTS US Cl XR 3,193,423 7/1965 Goifredo 156-48 15 .14;252 79,2

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent 3 ,758 ,351 Dated S p 11, 1973 Walter J. Striedieck and George Daniel Woodring Inventor(s) v It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as' shown below:

35 U.S .C. 254

Column 5, Example 6 line 53 change "Cu g./l 30.2"

to Cu' g./l 30. O- Y Column 5, Example 7 line '64, change "NaHAsO4 to Na 'HAsO Column 6, Example 8 line change" (19 /H O )v to (19% E 0) Column 6, Example ll, line 6 2 change ."Na NAs O to Na HAsO Column 9 Example 21, line 71, change "P0 to 'PO Column ll, Example 27 line. 29', change "NO to NO Column 13 line 15, change "anionic" to ionic I I Column 14, line48, change "thioxotropic" to thixotro pic Column 17, Claim 5, line 1, change "claim 4" to claim l- Signed and sealed this 25th day of :December 1973.

- (SEAL) Attest:

EDWARD M.PLETCHER,JR. RENE D. TEGTMEYER Attesting Off cer Acting Commissioner of Patents 

